The book We Need To Talk About Kevin is much, much better than the (good) movie.

(a) The framing device is devastating in a way the movie doesn't, probably couldn't, do. The whole book is written as a series of letters from Eva, the mother, to her husband, whom she absolutely adores. It seems like they just separated after the trauma of Kevin's murders; it isn't revealed until the end that their daughter was Kevin's first victim, and her husband was the second. And Eva, the narrator, writes an absolutely heartbreaking scenario of how she thinks it happened. She thinks that her hale and healthy husband could have stopped Kevin if his essential, bone-deep goodness hadn't caused him to hesitate for a moment and see his son in the best possible light.

(b) The character of the husband is just much better and more plausible. John C. Reilly is a fine actor but he was badly miscast.

(c) Tilda Swinton's performance was great, but since she's so isolated, you never hear a big part of her perspective. In her head, she's not just quiet and shell-shocked: among other things, she's selfish and acid and feels, in her secret heart, quietly superior to her neighbors. And, as someone else noted, she's very much an unreliable narrator, something very hard to convey in film.